Growing lines of evidence suggest that cardiac myocyte death, including apoptosis, is intimately involved in the pathogenesis of heart failure. Thus, an important therapeutic objective in reducing morbidity and mortality in patients with heart diseases is to prevent the cardiac myocyte loss, including apoptosis. We have recently identified that mammalian sterile 20 like kinase 1 (Mst1) plays an important role in mediating cardiac myocyte apoptosis and is intimately involved in the pathogenesis of congestive heart failure. Inhibition of endogenous Mst1 suppresses cardiac myocyte apoptosis in response to ischemia/reperfusion, doxorubicin-induced cardiotoxicity and cardiac remodeling, thereby reducing the size of myocardial infraction and improving long-term cardiac function in transgenic mice. Mst1 is unique among many pro-apoptotic molecules in that 1) Mst1 and caspase-3 stimulate a positive feed back mechanism, thereby powerfully amplifying apoptosis and that 2) Mst1 also inhibits cell growth response and mitochondrial functions, all of which should negatively affect cardiac function. Thus, we reasoned that Mst1 could be an important target to prevent cardiac myocyte death and progression of heart failure. Thus, our long-term goals are 1) to provide unequivocal evidence that inhibition of Mst1 is salutary for prevention of cardiac myocyte death in response to ischemia/reperfusion and heart failure in large animals in vivo, and to provide the rationale for the future efforts to develop small molecule inhibitors for Mst1, and 2) to identify the lead compound for the Mst1 inhibitor. We have recently identified that a 63 amino acid sequence derived from the C-terminus portion of Mst1 inhibits protein kinase activity of Mst1. Using this Mst1 -inhibitory peptide (Mst1-IP) as a tool, our goals in this Phase I studies are 1) to test whether extracellular treatment of cultured cardiac myocytes with the Mst1-IP conjugated with the cell permeable TAT sequence inhibits the kinase activity of Mst1 and blocks apoptosis in response to hypoxia/reoxygenation in cultured cardiac myocytes, 2) to narrow down the amino acid sequence required for the Mst1-IP to block the kinase activity of Mst1 3) and to develop a rapid assay system for determining the kinase activity of Mst1. The studies outlined here will be essential for us to conduct Phase II studies, in which we will obtain unequivocal evidence that the Mst1 inhibitor is useful for treatment of ischemia/reperfusion in large animals in vivo and identify the lead compound for further development of the Mst1 inhibitor.